U.S. patent number 7,268,687 [Application Number 10/807,072] was granted by the patent office on 2007-09-11 for radio frequency identification tags with compensating elements.
This patent grant is currently assigned to 3M Innovative Properties Company. Invention is credited to William C. Egbert, Thomas Herdtle.
United States Patent |
7,268,687 |
Egbert , et al. |
September 11, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Radio frequency identification tags with compensating elements
Abstract
Radio frequency identification (RFID) tags include compensating
elements. The compensating elements enhance the operation of a
compensated RFID tag, even when in close proximity to other RFID
tags, whether the other tags are compensated or uncompensated. The
compensating elements can include a closed loop of conductive
material added to a RFID tag antenna. The conductive loop
compensates the RFID tag performance when multiple RFID tags are in
close proximity, keeping the frequency response of the assembled
group of tags substantially centered near the operating frequency
of the RFID system.
Inventors: |
Egbert; William C.
(Minneapolis, MN), Herdtle; Thomas (Inver Grove Heights,
MN) |
Assignee: |
3M Innovative Properties
Company (St. Paul, MN)
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Family
ID: |
34961217 |
Appl.
No.: |
10/807,072 |
Filed: |
March 23, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050212707 A1 |
Sep 29, 2005 |
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Current U.S.
Class: |
340/572.7;
340/572.4 |
Current CPC
Class: |
G06K
19/07749 (20130101); G06K 19/07771 (20130101); H01Q
1/22 (20130101); H01Q 1/2225 (20130101); H01Q
7/00 (20130101) |
Current International
Class: |
G08B
13/14 (20060101) |
Field of
Search: |
;340/572.7,572.1,572.4,572.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 867 966 |
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Sep 1998 |
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EP |
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1 008 951 |
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Jun 2000 |
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EP |
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0 829 921 |
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Jun 2001 |
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EP |
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2003 087044 |
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Mar 2003 |
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JP |
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98/05088 |
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Feb 1998 |
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WO |
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98/31070 |
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Jul 1998 |
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WO |
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WO 02/15139 |
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Feb 2002 |
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WO |
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WO 03/096478 |
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Nov 2003 |
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WO |
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Other References
Carr, Joseph J., "Practical Antenna Handbook," 3.sup.rd Edition,
pp. 291-297 and 307-309, 1998. cited by other .
The ARRL Handbook for Radio Amateurs--The Standard in Applied
Electronics and Communications, 2001, pp. 20.36-20.39 and
20.68-20.69, 2000. cited by other .
"Multi-Loop Antenna For Radio Frequency Identification (RFID)
Communication", filed Feb. 20, 2004, U.S. Appl. No. 10/784,124.
cited by other .
"Multi-Loop Antenna For Radio-Frequency Identification", filed Mar.
3, 2003, U.S. Appl. No. 10/378,458. cited by other.
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Primary Examiner: Tweel, Jr.; John
Attorney, Agent or Firm: Buss; Melissa E.
Claims
The invention claimed is:
1. A compensating element comprising a closed loop of conductive
material having a size and proximity to an inductive loop antenna
of an RFID tag for electromagnetic coupling to the inductive loop
antenna to substantially maintain an operating frequency of the
inductive loop antenna at or near an operating frequency of an RFID
system in the presence of other RFID tags.
2. The compensating element of claim 1, wherein a parasitic current
is induced in the compensating element in response to the
electromagnetic coupling to the inductive loop antenna.
3. The compensating element of claim 1 wherein the conductive
material comprises one of a die cut metal foil, a patterned metal
foil, an electroplated conductive metal, a printed conductive ink,
and a printed precursor material reduced to a conductive state.
4. The compensating element of claim 1 having a substantially
rectilinear shape.
5. The compensating element of claim 1 having a substantially
circular shape.
6. The compensating element of claim 1 further comprising a
substrate upon which the closed loop is disposed.
7. The compensating element of claim 6 further comprising an
adhesive layer disposed on one side of the substrate.
8. The compensating element of claim 7 wherein the compensating
element and the adhesive layer are disposed on the same side of the
substrate.
9. The compensating element of claim 7 wherein the compensating
element and the adhesive layer are disposed on opposite sides of
the substrate.
10. A radio frequency identification (RFID) tag, comprising: an
inductive loop antenna; and a comnpensatixig element sized and
positioned on the RFID tag proximate the inductive loop antenna for
electromagnetic coupling to the inductive loop antenna to
substantially maintain an operating frequency of the inductive loop
antenna at or near an operating frequency of an RFID system in the
presence of other RFID tags.
11. The RFID tag of claim 10, wherein a parasitic current is
induced in the compensating element by a primary current in the
inductive loop antenna.
12. The RFID tag of claim 10 wherein the compensating element is
positioned for electromagnetic coupling to the inductive loop
antenna such that an RFID system interrogating antenna is able to
detect the compensated RFID tag when in close proximity to other
RFID tags.
13. The RFID tag of claim 10, further including a RFID die having
identification information stored therein.
14. The RFID tag of claim 10 wherein the compensating element
comprises a closed loop of conductive material.
15. The RFID tag of claim 14 wherein the closed loop has a
substantially rectilinear shape.
16. The RFID tag of claim 14 wherein the closed loop has a
substantially circular shape.
17. The RFID tag of claim 14 wherein the closed loop is
electrically isolated from the inductive loop antenna.
18. The RFID tag of claim 14 wherein the closed loop is
electrically connected to the inductive loop antenna.
19. The RFID tag of claim 14 wherein the closed loop is disposed
within an innermost loop of the inductive loop antenna.
20. The RFID tag of claim 14 wherein the closed loop is disposed
between loops of the inductive loop antenna.
21. The RFID tag of claim 14 wherein the closed loop is disposed
outside an outermost loop of the inductive loop antenna.
22. The RFID tag of claim 14 wherein the compensating element has
an angular displacement of between 0 and 45 degrees with respect to
an axis of the inductive loop antenna.
23. The RFID tag of claim 14 wherein the conductive material
comprises one of a die cut metal foil, a patterned metal foil, an
electroplated conductive metal, a printed conductive ink, and a
printed precursor material reduced to a conductive state.
24. The RFID tag of claim 14 wherein the closed loop is disposed
within 10 line widths of at least one loop of the inductive loop
antenna.
25. The RFID tag of claim 14 wherein the closed loop is disposed
within 2 line widths of at least one loop of the inductive loop
antenna.
26. The RFID tag of claim 10 wherein the compensating element has
an axis that is substantially aligned with an axis of the inductive
loop antenna.
27. The RFID tag of claim 10 wherein the compensating element lies
substantially in a plane parallel and proximate to a plane of the
inductive loop antenna.
28. The RFID tag of claim 10 wherein the compensating element is
substantially coplanar with the inductive loop antenna.
29. The RFID tag of claim 10 wherein the RFID tag resonates at a
frequency of approximately 13.56.+-.1.0 MHz.
30. The RFID tag of claim 10 wherein the compensating element is
physically separate from the inductive loop antenna.
31. The RFID tag of claim 10 wherein the compensating element
comprises at least one loop of the inductive loop antenna
electrically connected to at least one other loop of the inductive
loop antenna.
32. A radio frequency identification (RFID) tag, comprising: an
inductive loop antenna; and a compensating element positioned for
electromagnetic coupling to the inductive loop antenna, wherein the
compensating element comprises at least one loop of the inductive
loop antenna electrically connected to at least one other loop of
the inductive loop antenna, wherein the compensating element
comprises at least two loops of the inductive loop antenna, and
wherein each of the two loops of the inductive loop antenna is
electrically connected to a different one other loop of the
inductive loop antenna.
33. The RFID tag of claim 32 wherein the at least two loops of the
inductive loop antenna electrically connected to at least one other
loop of the inductive loop antenna are adjacent loops.
34. The RFID tag of claim 32 wherein the at least two loops of the
inductive loop antenna electrically connected to at least one other
loop of the inductive loop antenna are non-adjacent loops.
35. A radio frequency identification (RFID) tag, comprising: an
inductive loop antenna; and a compensating element positioned for
electromagnetic coupling to the inductive loop antenna, wherein the
compensating element comprises at least one loop of the inductive
loop antenna electrically connected to at least one other loop of
the inductive loop antenna, and wherein the at least one loop of
the inductive loop antenna is electrically shorted to the at least
one other loop of the inductive loop antenna.
36. A Radio Frequency Identification (RFID) tag for placement on a
conductive surface, comprising: a substrate; an inductive loop
antenna positioned on the substrate; a compensating element sized
and positioned on the RFID tag proximate the inductive loop antenna
for electromagnetic coupling to the inductive loop antenna to
substantially maintain an operating frequency of the inductive loop
antenna at or near an operating frequency of an RFID system in the
presence of other RFID tags; and a dielectric spacer positioned
between the substrate and the conductive surface.
37. The RFID tag of claim 36 wherein the dielectric spacer has a
dielectric constant less than 10.
38. The RFID tag of claim 37 wherein the dielectric spacer has a
dielectric constant less than 3.
39. The RFID tag of claim 36 wherein the dielectric spacer has a
thickness of less than 10 mm.
40. The RFID tag of claim 36 wherein the dielectric spacer has a
thickness of less than 5 mm.
41. The RFID tag of claim 10, wherein the inductive loop antenna is
a multi-turn inductive loop antenna.
42. The RFID tag of claim 10, wherein the operating frequency of
the inductive loop antenna is inversely proportional to a distance
between the compensating element and the inductive loop
antenna.
43. The RFID tag of claim 10, wherein the inductive loop antenna is
electromagnetically coupled to an interrogating magnetic field
generated by an RFID reader, and wherein the compensating element
is not electromagnetically coupled to the interrogating magnetic
field generated by the RFID reader.
44. The RFID tag of claim 24, wherein the RFID tag resonates at a
frequency of approximately 13.56.+-.1.0 MHz.
45. A method comprising: selecting a size for a compensating
element; forming the compensating element according to the selected
size; and positioning the compensating element on an RFID tag
proximate an inductive loop antenna so as to provide
electromagnetic coupling by the compensating element to the
inductive loop antenna to substantially maintain an operating
frequency of the inductive loop antenna at or near an operating
frequency of an RFID system in the presence of other RFID tags.
46. The method of claim 45, wherein the conductive loop antenna is
a multi-turn antenna, and wherein positioning the compensating
element comprises positioning the compensating element interspersed
with loops of the inductive loop antenna.
47. The method of claim 45, further comprising electrically
connecting the compensating element to antenna via a conductive
jumper connecting an innermost loop of the inductive loop antenna
to a point on a perimeter of the compensating element.
48. The method of claim 45, wherein selecting the size of the
compensating element comprises selecting a diameter of the
compensating element based on a diameter of the inductive loop
antenna, and wherein a frequency response of the inductive loop
antenna is greater when the diameter of the compensating element is
sized within a range bounded by a diameter of an innermost loop of
the inductive loop antenna and a diameter of an outermost loop of
the inductive loop antenna than when the diameter of the
compensating element is not sized within the range.
Description
TECHNICAL FIELD
The invention relates to the use of radio frequency identification
systems for document and file management and, more specifically to
radio frequency identification tags for radio frequency
identification systems.
BACKGROUND
Radio-Frequency Identification (RFID) technology has become widely
used in virtually every industry, including transportation,
manufacturing, waste management, postal tracking, airline baggage
reconciliation, and highway toll management. A typical RFID system
includes RFID tags, an RFID reader having an antenna, and a
computing device. The RFID reader includes a transmitter that may
provide energy or information to the tags, and a receiver to
receive identity and other information from the tags. The computing
device processes the information obtained by the RFID reader.
In general, the information received from the tags is specific to
the particular application, but often provides identification for
an article to which the tag is fixed, which may be a manufactured
article, a vehicle, an animal or individual, or virtually any other
tangible article. Additional data may also be provided for the
article. The tag may be used during a manufacturing process, for
example, to indicate a paint color of an automobile chassis during
manufacturing or other useful information.
The transmitter outputs RF signals through the antenna to create an
electromagnetic field that enables the tags to return an RF signal
carrying the information. In some configurations, the transmitter
initiates communication, and makes use of an amplifier to drive the
antenna with a modulated output signal to communicate with the RFID
tag. In other configurations, the RFID tag receives a continuous
wave signal from the RFID reader and initiates communication by
responding immediately with its information.
A conventional tag may be an "active" tag that includes an internal
power source, or a "passive" tag that is energized by the field. In
either case, the RFID tags communicate using a pre-defined
protocol, allowing the RFID reader to receive information from one
or more tags. The computing device serves as an information
management system by receiving the information from the RFID
reader, and performing some action, such as updating a database or
sounding an alarm. In addition, the computing device serves as a
mechanism for programming data into the tags via the
transmitter.
SUMMARY
Radio frequency identification (REID) tags include compensating
elements. The function of the compensating element becomes
discernable when a compensated REID tag is in the presence of a
group of other REID tags. The compensating element increases the
likelihood that the compensated RED tag will be detected by an REID
system, even when in close proximity to other RED tags, whether the
other REID tags are similarly compensated, differently compensated,
or uncompensated.
The compensating element can include a closed loop of conductive
material placed substantially proximate to the RFID tag antenna. In
use, the compensating element is electromagnetically coupled to the
RFID tag antenna such that the primary current induced in the RFID
antenna induces a counter-circulating parasitic current in the
compensating element. This parasitic current results in reduced
tag-to-tag coupling between the compensated RFID tag and the other
RFID tags in the group.
The details of one or more embodiments of the invention are set
forth in the accompanying drawings and the description below. Other
features, objects, and advantages of the invention will be apparent
from the description and drawings, and from the claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram illustrating a radio frequency
identification (RFID) system in which RFID tags incorporate
compensating elements in accordance with the techniques described
herein.
FIG. 2 is a schematic diagram of one example embodiment of a
compensated RFID tag according to the techniques described
herein.
FIGS. 3-10 are schematic diagrams of additional example embodiments
of compensated RFID tags.
FIGS. 11A-11C are side view perspective diagrams of additional
example embodiments of compensated RFID tags.
FIGS. 12A-12C are side view perspective diagrams of additional
example embodiments of compensated RFID tags.
FIG. 13 is a diagram generally illustrating the direction of the
currents in a compensated RFID tag.
FIG. 14 is a graph showing example response of five compensated
RFID tags in the presence of an interrogating field.
FIG. 15A is a graph showing resonant frequency of a rectilinear
compensated RFID tag versus size of the compensating element.
FIG. 15B is a graph showing resonant frequency of a rectilinear
compensated RFID tag versus size of the compensating element, and
of a circular compensated RFID tag versus size of the compensating
element.
FIG. 16 is a graph showing resonant frequency of a compensated RFID
tag versus angular displacement of the compensating element.
FIG. 17 is a graph showing resonant frequency and amplitude versus
various shorted coil combinations in a compensated RFID tag.
FIG. 18 is a diagram showing a RFID tag with a compensating element
on a conductive substrate.
DETAILED DESCRIPTION
FIG. 1 is a block diagram illustrating an example RFID system 10 in
which RFID tags incorporate compensating elements in accordance
with the techniques described herein. In the illustrated example of
FIG. 1, RFID system 10 is used to track books, documents, files or
other articles. The RFID system may, for example, be deployed
within libraries, law offices, government agencies, or other
facilities that generate and/or store documents and files, such as
business, criminal, and medical records. The articles contain RFID
tags that uniquely identify the articles. In addition, each RFID
tag may also contain information describing the article, and status
information indicating whether removal of the article is
authorized. The RFID tags may be embedded within the articles so
that the tags are substantially imperceptible, thereby reducing or
prevent tampering.
As illustrated in FIG. 1, RFID system 10 includes an exit control
system 15 that detects unauthorized removal of articles from a
protected area. For example, the protected area may be a library
and the articles may be books or other articles that are generally
checked out from and back into the library. The techniques could
also be applied to other kinds of articles without departing from
the scope of the present invention.
Exit control system 15 includes lattices 19A and 19B which define
an interrogation zone or corridor located near the exit of
protected area. The lattices 19A and 19B include antennas for
interrogating the RFID tags as they pass through the corridor to
determine whether removal of the article to which the tag is
attached is authorized. Exit control system 15 may utilize at least
one RFID reader (not shown) to drive the antennas. To detect a tag,
the RF reader outputs RF power through the antennas to create an
electromagnetic field within the interrogation corridor. In
general, the terms "electromagnetic field" and "magnetic field" are
used interchangeably herein as the magnetic component is used to
couple with the RFID tags.
The RF reader receives information from any tags present within the
interrogation corridor, and exit control system 15 determines
whether removal of the article is authorized. If removal of the
article is not authorized, exit control system 15 initiates some
appropriate security action, such as sounding an audible alarm,
locking an exit gate, or other action.
In addition, RFID system 10 includes a check-in/check-out area 11
by which an authorized person processes articles for removal or
return. In particular, check-in/check-out area 11 includes an RFID
reader 18 for interrogating RFID tags fixed to articles and
changing their status as desired, e.g., checking-in or checking-out
the articles. The check-in/check-out area 11 may be used, for
example, to check file folders out of a file room or to check books
out of library.
In addition, articles may be positioned in a number of storage
areas e.g., on an open shelf 12A, a cabinet 12B, a vertical file
separator 12C (collectively, "storage areas 12") or other location,
as shown in FIG. 1. Each storage area 12 includes tag interrogation
capability which enables tracking of articles throughout a
facility. File folders in an office or medical setting, for
example, could be tracked throughout the facility via storage areas
12. In a library setting, for example, a book could be tracked
after check-in while on shelf 12A.
To provide RFID interrogation capability, each article stored
within storage areas 12 has an associated RFID tag. The tag may be
embedded within the article or applied to the article or to the
packaging of the article so that the tag is at least substantially
imperceptible, which can help to prevent detection and tampering.
The RFID tag may be applied at the end user location, or may be
inserted into or applied to an article during its manufacture, as
with a file folder, document, book, or the like.
Individual tags for RFID systems operating in the high-frequency
(HF) range, e.g., greater than 3 megahertz (MHz), of the
electromagnetic spectrum typically use inductive loop antennas,
with diametric sizes ranging from a few millimeters (mm) to a few
tens of mm. A silicon die attached to the inductive loop antenna
provides electronic functions that may include signal receive and
send, data processing, unique identification information, and data
storage and retrieval. RFID readers or interrogators located within
RFID system 10, e.g., at exit control system 15, check-in/check-out
station 11, and storage locations 12, use antennas to communicate
with the RFID tags by electromagnetic (wireless) transmitted and
received signals. The RFID readers in turn communicate with an
article management system 14, either through a wireless link or a
wired cable connection.
The RFID tag may be powered by an electrochemical battery (a
so-called "active tag"), or the RFID tag may draw its power
entirely from the RF field emanating from the reader (a so-called
"passive tag"). In the latter case the RFID tag may remain
unpowered and dormant indefinitely, with no requirement for
maintenance of batteries or external power supplies. Although the
following discussion will focus mainly on passive (i.e.,
battery-less) RFID tags, it shall be understood that the invention
is not limited to passive tags, and that the principles and results
described herein are also applicable to active HF RFID tags with
inductive loop antennas.
REID system 10 may operate in a band of the electromagnetic
spectrum defined by governmental regulations for electromagnetic
radiation emissions. For example, REID system 10 may operate at a
common worldwide standard in the Industrial-Scientific-MediCal
(ISM) band centered at 13.56 MHz with an allowable frequency
variance of +/-7 kHz. However, other frequcncies may be wed for
REID applicatioDs, and the invention is not so limited. For
example, some REID systems in large storage areas such as a
warehouse may use an RFID system that operates at approximately 900
MHz. It shall be understood that one skilled in the art could
reasonably extend the operation of RFID system 10 to other
frequencies, for example, inductive loop RFID antennas operating at
frequencies other than 13.56 MHz in the HF band, and to other
bands, e.g., the Low Frequency (LF) band at 125 kHz to 138 kHz.
The antennas of the readers and interrogators located within RFID
system 10 typically couple to the RFID tags through near-field
magnetic induction. A time-varying RF field produced by a reader,
for example, couples by magnetic induction to a loop antenna on the
RFID tag, inducing an electromotive force ("voltage") in the
conductive loop or loops of the RFID tag antenna. The induced
electromotive force drives electrical currents through the RFID tag
antenna. The electrical power received by the RFID tag antenna is
converted by the RFID die to the electrical voltage required to
operate the internal circuits of the die. The reader communicates
with the RFID die by suitable modulation of the carrier frequency.
The die communicates with the reader by modulating the load it
presents to the RFID tag antenna, causing modulated back-scatter of
the RF field surrounding the RFID tag. The reader's receiver
detects the back-scattered signal from the RFID tag. The distance
at which the reader can reliably communicate with the tag, the
"read range", is a function of reader design, radiated power, RFID
die design, RFID tag antenna, and reader-tag antenna
orientation.
To achieve the maximum read range, the RFID tags can be tuned to be
electrically resonant near the operating frequency of the RFID
system. Tuning to the system operating frequency supports maximum
energy transfer from the RF field to the RFID tag.
The RFID interrogators or readers communicate position information
to article management system 14 that provides a central data store
for aggregation of the position information. Article management
system 14 may be networked or otherwise coupled to one or more
computers so that individuals at various locations can access data
relative to those articles.
Collection and aggregation of the position information may be
useful for a number of purposes. For example, a user may request
the location of a particular article or group of articles, such as
a file or a group of books. Article management system 14 may
retrieve position information from the data store, and report to a
user the last location at which the articles were located within
one of the storage areas. Optionally, article management system 14
can re-poll or otherwise re-acquire the current location of the
article to verify that the article is in the location indicated in
the database.
As mentioned above, each of the storage areas 12 of system 10 may
be equipped with one or more reader antennas for interrogating the
articles to aid in determining which articles are located at each
of the storage areas. One example reader antenna which may be used
is described in copending and commonly assigned U.S. patent
application Ser. No. 10/378,458 filed Mar. 3, 2003, the entire
content of which is incorporated herein by reference. One or more
antennas may be positioned within open shelf 12A to create an
electromagnetic field for communicating with the RFID tags
associated with the articles stored therein. Similarly, antennas
may be located within cabinet 12B, vertical file separator 12C,
desktop reader, and or other location. The antennas may be
positioned in various ways, such as on top or bottom of each shelf,
at the back of the shelves, or supported vertically, interspersed
among the files. The antennas can be retrofitted to existing
shelves or built into a shelf and purchased as a unit. The system
may be configured to interrogate, or poll, the RFID tags in any
number of ways. For example, the antennas may poll the RFID tags
continuously, poll the tags in a sequence specified by the user, or
poll the tags on demand.
Often, a group of articles with RFID tags, such as file folders on
a shelf, are located in close proximity in a reader or interrogator
of RFID system 10. Conventional RFID tags, including tags tuned for
optimum function at the RFID system operating frequency f.sub.0,
would tend to show significant interference, i.e., tag-to-tag
coupling, when in close proximity to each other. This interference
results in an inability to "read" or identify some or all of the
individual RFID tags in the group. As a result, accurate or up to
date information as to the location of each individual article
tagged with conventional RFID tags may not be obtained.
In contrast, RFID system 10 utilizes "compensated RFID tags" tat
incorporate compensating elements 30. Compensated RFID tags arc
useful, for example, where it may be desirable to read a group of
RFID tags that are in close proximity to each other. For example,
RFID) tags attached to file folders or books may be in close
proximity to other RFID tags when the articles containing the tags
are stored on a shelf or in a drawer, or carried through an exit
control system. The compensated RFID tags are designed such that
each compensated RFID tags, may be read individually as well as
when it is in close proximity to other RFID tags, regardless of
whether the other RFID tags are similarly compensated, differently
compensated, or uncompensated.
Various example embodiments of compensating elements and
compensated RFID tags will now be given with respect to FIGS. 2-12.
A detailed description of the operation of various example
embodiments of compensating elements and of compensated RFID tags
is given in more detail below with respect to FIGS. 13-17.
FIG. 2 is a schematic diagram illustrating an example embodiment of
an RFID tag 20 having a compensating element 30. An RFID tag 20
having a compensating element will be referred to herein generally
as a "compensated RFID tag 20" or "compensated RFID tag".
Conventional, uncompensated RFID tags known in the art will be
referred to as "uncompensated RFID tags." For ease of illustration,
compensating element 30 is shown in FIG. 2 as lying entirely within
innermost loop 24A of the antenna 24. It shall be understood,
however, that the specific location of compensating element 30
shown in the embodiment of FIG. 2 is one of many embodiments where
the compensating element may be located with respect to antenna 24,
and that the invention is not limited in this respect. Alternate
embodiments of compensated RFID tags 20 and compensating elements
30 will be shown and described in more detail below.
A substrate 22 provides support for antenna 24, compensating
element 30 and the other components of compensated RFID tag 20.
Antenna 24 is a multi-turn inductive loop antenna having multiple
loops, including innermost loop 24A and outermost loop 24B.
Although antenna 24 is shown throughout the FIGURES as a multi-turn
inductive loop antenna, it shall be understood that antenna 24 may
have a single loop, or may also have more or fewer loops than are
explicitly shown in the FIGURES, and that the number of loops on
antenna 24 is not to be taken as limiting. Antenna 24 may be formed
on substrate 22 by any of several conductive pattern technologies,
or may be formed separately and transferred to the substrate. One
or more tuning capacitors (not shown) may be connected to antenna
24 to form an electrical resonant circuit. The multiple loops of
antenna 24 are closed through one or more via connections 28. RFID
die 26 may be connected to antenna 24 using any one of several
interconnection technologies, such as conductive adhesives, solder,
or metal-to-metal contact.
In the embodiment shown in FIG. 2, compensating element 30 is a
closed loop of conductive material. In one embodiment, the
compensating element 30 lies substantially in a plane parallel and
proximate to the plane of antenna 24 for which it provides
compensation. In another embodiment, compensating element 30 lies
substantially in the same plane of (i.e., is substantially coplanar
with) antenna 24 for which it provides compensation. These and
other embodiments of compensated RFID tags 20 and of compensating
elements 30 will be described in further detail below.
The embodiment of the compensating element 30 shown in FIG. 2 is
substantially rectilinear in shape and is of similar shape to
antenna 24. However, it shall be understood that the compensating
element 30 may take many other shapes, and that it need not be
similarly shaped to antenna 24. The term "closed loop" therefore
can be defined as any shape closed upon itself, for example,
square, rectangle, circle, ellipse, triangle, any other
multi-sided, or smoothly-sided shape closed upon itself such that
an electrical current can flow in the loop. These and other
embodiments of the compensating element 30 and compensated RFID tag
20 will be readily understood to those of skill in the art upon
reading and understanding the present specification.
The function of the compensating element 30 is relevant when the
compensated RFID tag 20 is in the presence of at least one other
RFID tag. In use, the compensating element 30 is
electromagnetically coupled to the RFID tag antenna 24 such that
the primary current induced in the RFID antenna 24 induces a
counter-circulating parasitic current in the compensating element
30. This parasitic current results in reduced tag-to-tag coupling
between the compensated RFID tag and the other RFID tags in the
group. The compensating element 30 thus increases the likelihood
that the compensated RFID tag 20 will be detected by the RFID
system 10, even when in close proximity to other RFID tags, whether
the other RFID tags are similarly compensated, differently
compensated, or uncompensated. Operation of the compensating
element 30 will be described in more detail below with respect to
FIGS. 13-17.
The compensating element 30 may be formed in any one of several
ways. One method is to form the compensating element 30 as part of
the RFID tag antenna 24 during manufacture, using the same
operations that are used to manufacture the basic antenna
structure. Examples of circuit-forming operations include but are
not limited to, die cutting or patterning metal foil,
electroplating conductive metals, printing conductive inks,
printing precursor materials (e.g., metallo-organic compounds) that
are reduced to a conductive state by subsequent heating or drying,
and the like. The substrate 22 may be a polymer film, paper, rigid
plastic film, electronic circuit board, or other similar
nonconductive materials.
Another approach is to form the compensating element 30 in a
manufacturing operation, separate from the antenna manufacture, on
the first or second surface of the RFID antenna 24, using either
the same process used to create the patterned conductive antenna
24, or using a different process.
Yet another approach is to form the compensating element 30 as a
separate circuit by any of the diverse conductive pattern forming
techniques noted previously. The compensating element 30 may be
placed in close proximity to, but not attached to, the RFID tag 20.
Or, the compensating element 30 may be attached to the RFID tag
antenna 24 to form a single unit, using, for example, an adhesive
film, curable adhesive pastes, double-sided pressure sensitive
adhesive tape, or the like, to create a suitable configuration of
the compensating element 30 proximate to the RFID tag antenna
24.
FIG. 3 shows another embodiment of a compensated RFID tag 20 with a
compensating element 30. Again, for ease of illustration, the
compensating element 30 is shown, in this embodiment, as lying
entirely within the innermost loop 24A of antenna 24. In FIG. 3,
the compensating element 30 is electrically connected to antenna 24
via a conductive jumper (short circuit) 32 connecting innermost
coil 24A to the compensating element 30 at one point on the
perimeter of the compensating element 30. In other words, the
compensating element 30 may be in electrical contact with the RFID
tag antenna 24 and still perform the compensation function
described herein.
In FIG. 2, the compensating element 30 was not electrically
connected to the antenna 24. It shall be understood that the
compensating element 30 may be electrically connected to the RFID
tag antenna 24, or it may be located proximate to, but electrically
isolated from, the RFID tag antenna 24. Either arrangement, whether
the compensating element 30 is electrically connected to or
electrically isolated from the RFID tag antenna 24, results in a
compensating element 30 can have the compensating effect described
herein.
FIG. 4 shows another example embodiment of a compensated RFID tag
20. There, compensating element 30 is interspersed between loops of
antenna 24. The compensating element 30 could, in fact, be located
between any of the loops of the antenna 24 and still have the
compensating effect described herein.
FIG. 5 shows another embodiment of a compensated RFID tag 20. In
this embodiment, the compensating element 30 is located outside of
outermost loop 24B of antenna 24. The embodiments shown in FIGS.
2-5 demonstrate that the compensating element 30 may be located
entirely inside of, interspersed with, or entirely outside of the
loops of the antenna 24 without departing from the scope of the
present invention.
FIG. 6 shows a compensated RFID tag 20 including a compensating
element 30 having an axis that is substantially aligned with the
axis of the antenna 24. That is, the lines of the compensating
element 30 are substantially parallel to the corresponding lines of
antenna 24.
FIG. 7 shows another embodiment of a compensated RFID tag 20 having
a compensating element 30 having an axis at an "offset angle" 34 of
approximately 45 degrees with respect to the axis of antenna 24. In
some RFID tag applications, an embodiment such as that shown in
FIG. 6 may be the used. For example, when building the compensating
element 30 into an RFID tag at the time of manufacture, substantial
alignment of the compensating element 30 and the RFID tag antenna
24 can be ensured. In that case, the compensating element 30 can
simply be incorporated into the artwork for the production of the
RFID tag 20 itself, and substantial alignment of compensating
element 30 with the axis of the RFID tag antenna 24 can be
achieved.
In other applications, an embodiment such as that shown in FIG. 7
may be appropriate. For example, when adding compensating elements
30 to conventional, uncompensated RFID tags, the compensating
elements 30 may be placed, due to human error or by design, at an
offset angle 34 with respect to the axis of antenna 24. It shall be
understood, therefore, the angle of placement of the compensating
element 30 with respect to the axis of antenna 24 is not a limiting
factor for purposes of the present invention, and that any angular
placement of the compensating element 30 with respect to the axis
of antenna 24 is within the scope of the present invention.
FIG. 8 shows another example embodiment of a compensated RFID tag
20. In this example embodiment, compensating element 30 is circular
in shape, rather than rectilinear in shape as those described above
with respect to FIGS. 2-7. Indeed, the compensating element 30 may
take virtually any other shape, including triangular, elliptical,
square, rectangular, or any of a myriad of other multi-sided or
smoothly-sided closed shapes and still perform the compensating
function. It shall be understood, therefore, that the shape of the
compensating element 30 is not a limiting factor for purposes of
the present invention, and that the compensating element 30 may
take virtually any shape without departing from the scope of the
present invention. The placement of a circular compensating element
20 such as that shown in FIG. 8 will not demonstrate angular
dependence, since the circular loop is circumferentially symmetric
around the center of the loop. The effect of the compensating
circuit element 20 will be maximized when the geometric center of
the compensating circuit element 20 is coincident with the
geometric center of the RFID tag antenna 24.
FIG. 9A shows a top view and FIG. 9B shows a side view of an
example embodiment of a supplemental compensating article 21.
Supplemental compensating article 21 includes a compensating
element 30 located on a top side of substrate 23, with an adhesive
surface 25 disposed on the opposite side of the substrate 23.
Supplemental compensating article 21 can be used to add
compensation to conventional, uncompensated RFID tags in the manner
shown in FIG. 10. Adhesive surface 25 can be an adhesive film,
curable adhesive pastes, double-sided pressure sensitive adhesive
tape, or the like.
FIG. 10 shows perspective view of a conventional, uncompensated
RFID tag 33 with an RFID antenna 35 deposited on a substrate 37. To
improve the performance of this uncompensated RFID tag 33, a
supplemental compensating article 21 can be adhered to the
uncompensated RFID tag 33. The adhesive surface 25 on the underside
of the substrate 23 of the supplemental compensating article 21 is
brought into contact with the uncompensated RFID tag 33. In the
case of a non-rotationally independent shape (e.g., square,
rectangle, ellipse, etc.) the compensating element 30 could be
substantially aligned with or placed at an off set angle with
respect to the axis of the RFID antenna 35. Alternatively, the
adhesive surface 25 could be placed between the compensating
element 30 and the substrate 23, or on top of the compensating
element 30. The supplemental compensating article 21 would then be
attached appropriately. Use of the supplemental compensating
article 21 allows users of conventional, uncompensated RFID tags to
easily add compensating elements to their conventional,
uncompensated RFID tags without requiring the purchase of an
entirely new set of tags.
FIGS. 11A, 11B and 11C are side views of example embodiments of
compensated RFID tags 20. Each of FIGS. 11A, 11B, and 11C shows a
compensated RFID tag 20 having a substrate 22 and an RFID antenna
24. The compensating element 30 in each of FIGS. 11A-11C is located
substantially in the plane of antenna 24. The compensating element
30 may be located entirely within innermost loop 24A (FIG. 11A),
interspersed between loops (FIG. 11B), or located entirely outside
of outermost loop 24B (FIG. 11C).
FIGS. 12A, 12B and 12C are side views of additional example
embodiments of compensated RFID tags 20. Each of FIGS. 12A, 12B,
and 12C shows a compensated RFID tag 20 having a substrate 22 and
an RFID antenna 24. The compensating element 30 in each of FIGS.
12A-12C is located in a plane substantially parallel and proximate
to the plane of antenna 24. In these views, the compensating
element 30 may be, but need not be physically separated by a
substrate layer 23. Again, the compensating element 30 may be
located entirely within innermost loop 24A (FIG. 12A), interspersed
between loops (FIG. 12B), or located entirely outside of outermost
loop 24B (FIG. 12C).
Now consider a group of articles, e.g., file folders, located on a
shelf and marked with conventional, uncompensated RFID tags. When a
conventional, uncompensated RFID tag is in close proximity to other
RFID tags, as they could be in a group of shelved folders or
similar articles, the electromagnetic field from the first
uncompensated RFID tag interacts with and couples to other nearby
RFID tags. The effective resonance frequency of the collection of
interacting uncompensated RFID tags is shifted downward and may
shift outside of the bandwidth of operation of the RFID system.
When the resonant frequency of the group of uncompensated RFID tags
is shifted away from the system operating frequency the
communication between the reader and the group of uncompensated
RFID tags may be degraded or lost entirely.
The compensating element 30 of a compensated RFID tag 20 modifies
the effective inductance L of the compensated RFID tag antenna 24.
The resonant frequency f.sub.TAG of the compensated RFID tag 20 is
less affected by the near physical presence of other RFID tags.
This is true for each compensated RFID tag 20 in the group,
regardless of whether the other RFID tags in the group are
similarly compensated, differently compensated, or
uncompensated.
The compensated RFID tag 20 can be tuned so that its resonant
frequency f.sub.TAG is centered near the operating frequency
f.sub.0 of RFID system 10 so that it may be read in isolation from
other RFID tags. When the compensated RFID tag 20 is one of a group
of other RFID tags, whether compensated or not, the compensated
RFID tag response f.sub.TAG remains tuned near the system operating
frequency. When the compensated RFID tag 20 is one of a group of
compensated RFID tags, the compensated RFID tag response for each
compensated tag remains tuned near the system operating frequency,
and the group response f.sub.GROUP also remains tuned near the
system operating frequency. In this way, the likelihood that the
RFID system 10 will detect presence of a particular RFID tag in the
group is increased when that tag is a compensated RFID tag,
regardless of whether the other tags in the group are similarly
compensated, differently compensated, or uncompensated. Similarly,
the likelihood that the RFID system 10 will detect presence of all
of the tags in the group is increased when all of the tags in the
group are compensated RFID tags.
FIG. 13 shows a diagram generally illustrating the direction of the
currents circulating in a compensated RFID tag. In operation, the
magnetic flux from the external magnetic interrogating field
generated by the RFID reader of RFID system 10 induces a primary
current, indicated generally by line 42, in the antenna 24 flowing,
in the example of FIG. 13, in a counter-clockwise direction. This
primary current 42 induces, by virtue of electromagnetic coupling,
a counter-circulating parasitic current, indicated generally by
line 44, in the compensating element 30. The results of the induced
parasitic current 44 include a lowered effective inductance for the
RFID tag antenna 24, an increased resonant frequency f.sub.TAG, a
reduced response, or sensitivity, to the magnetic field applied by
the RF reader, and a reduction in tag-to-tag coupling. The overall
result is that RFID system 10 is more likely to detect each
compensated RFID tag 20 in a group of RFID tags.
FIG. 14 shows responses for compensated RFID tags 20. Curve 40
indicates the response of RFID system 10. The single-tag
compensated RFID tag response (curve 52) was tuned to be centered
around 14.5 MHz, slightly higher than the system operating
frequency f.sub.0=13.56 MHz. It shall be understood, however, that
a compensated RFID tag 20 could be tuned nearer to 13.56 MHz, for
example, 13.56.+-.1 MHz. When a second compensated RFID tag is
aligned coaxially within 0.5 inch separation from the first
compensated RFID tag, the resonant frequency of the pair is shifted
down to 13.8 MHz as shown by curve 54. When five compensated RFID
tags are brought together as shown by curve 56, the center
resonance frequency of the group, 14.0 MHz, is nearly unchanged
from the pair resonance (curve 54), and the response amplitude is
within 0.5 dB of the single compensated RFID tag response (curve
52). When ten compensated RFID tags are stacked, the response curve
58 shows that the group resonance frequency peaks at 14.3 MHz,
nearly unchanged from the two- and five-compensated RFID tag cases.
Thus, each of the compensated RFID tags in the group is
individually readable with a single RFID reader. FIG. 14
illustrates that the compensating element 30 results in compensated
RFID tags in which one, two, or many tags may be read with an RFID
reader operating at the system operating frequency.
The addition of compensating elements 30 to an inductively coupled
RFID tag antenna 24 modifies the interaction of the compensated
RFID tag 20 with the magnetic field component of the RF energy
field generated by the antenna 24 incident on the compensating
element 30. The inductance L characterizes the coupling between the
current induced in the RFID tag antenna 24 and the magnetic flux
through the antenna 24. The magnetic flux is a function of the
magnetic field B, the area of the antenna A, and the number of
turns N in the antenna. The magnetic field B is the vector sum of
the fields created by the reader, the induced electrical current in
the RFID tag, and the electrical currents in neighboring RFID tags.
The compensating element 30 contributes to the net current flowing
in the compensated RFID tag and "compensates" for the presence of
the neighboring RFID tags, whether compensated or uncompensated, by
reducing the apparent inductance L of the RFID antenna 24.
FIG. 15A shows a graph of the frequency and amplitude response of a
compensated RFID tag 20 vs. size of the compensating element 30.
The results were taken from a standard, uncompensated RFID tag
modified to include rectilinear compensating elements 30 of varying
size. The diameter of the innermost loop of the RFID tag under test
was approximately 25 mm, and the diameter of the outermost loop was
approximately 45 mm. As shown in FIG. 15A, for compensating element
30 diameters of less than 22.5 mm, the compensating element 30 had
no effect on the response of the RFID tag under test. However, once
the diameter of the compensating element 30 became close to the
diameter of the innermost loop of the tag under test, a measurable
effect can be seen. Namely, the frequency response is shifted
upwards, with a maximum when the diameter of the compensating
element 30 approaches 40 mm.
FIG. 15B shows a graph of the frequency and amplitude response of a
compensated RFID tag 20 vs. size for a circular compensating
element 30. Results for similarly sized rectilinear compensating
elements 30 are also shown. As with the rectilinear compensating
element, once the diameter of the compensating element becomes
close to the diameter of the innermost loop of the tag under test,
a measurable effect can be seen. Again, the frequency response is
shifted upwards, with a maximum when the diameter of the circular
compensating element and also the square compensating element is
about 40 mm. From FIGS. 15A and 15B we can infer that the
compensating element 30 may take virtually any shape and still
perform the compensating function described herein.
FIGS. 15A and 15B also indicate that the effect of the compensating
element 30 is a proximity coupled effect. That is, the compensating
element 30 is proximity coupled to the RFID tag antenna 24. The
compensating element 30 carries a coupled parasitic current driven
by the current in the RFID tag antenna 24. The compensating element
30 is thus electromagnetically coupled to the RFID tag antenna 24
as opposed to the interrogating magnetic field generated by the
RFID reader antenna. To produce this proximity coupled effect, the
compensating element 30 can be positioned for electromagnetic
coupling to the RFID tag antenna, i.e., such that a primary current
in the antenna 24 induces a parasitic current in the compensating
element 30. The compensating element 30 can be placed within 10
conductor line widths of at least one loop of antenna 24 to be
positioned for electromagnetic coupling to the RFID tag antenna 24
to produce the proximity coupled effect. The term "conductor line
width" refers to the line width of the proximity coupled loop or
loops of antenna 24. Stronger proximity coupling is achieved when
the compensating element 30 is positioned relatively closer to at
least one loop of antenna 24, such as when the compensating element
30 is placed within 1-2 conductor line widths of at least one loop
of antenna 24. However, it shall be understood that, as long as the
compensating element 30 and the antenna 24 are positioned for
electromagnetic coupling, the precise distance at which they are
spaced is not a limiting factor for purposes of the present
invention.
FIG. 16 shows the effect of angular displacement for a
substantially rectilinear compensating element 30 with respect to
the axis of a substantially rectilinear antenna 24. The
compensating element was chosen to have an edge length
approximately equal to the average edge length of the multiple
loops of the RFID tag antenna. As shown, the proximity coupling
effect is greatest at an angular displacement, or offset angle,
(see FIGS. 6 and 7) of 0 degrees. As the angular displacement
increases, the proximity coupling effect decreases nonlinearly
until an angular displacement of approximately 10 degrees is
reached. For angular displacements beyond 10 degrees, the proximity
coupling effect decreases slowly for angular displacements up to 45
degrees.
FIG. 16 demonstrates that the proximity coupled effect on f.sub.TAG
is greatest at an angular displacement of 0 degrees, and then falls
off as the angular displacement is increased up to approximately 10
degrees. For angular displacements of greater than 10 degrees, any
additional change in angular displacement has less effect on the
RFID tag response. This means that, for those applications where
compensating elements 30 are attached to conventional,
uncompensated RFID tags, a more stable, predictable result may be
achieved if compensating elements 30 are attached to the
uncompensated RFID tags at an angle, rather than "square" with the
loops of antenna 24. It shall be understood that although FIG. 16
was measured with respect to substantially square compensating
element and RFID antenna 24, similar results would be obtained for
other non-rotationally independent shapes. For rotationally
independent shapes (e.g., a circle) the angular displacement should
have no effect.
Additional embodiments of compensated RFID tags 20 and compensating
elements 30 will now be described with respect to FIG. 17. FIG. 17
shows the frequency response f.sub.TAG 101 and amplitude 103 of
compensated RFID tags with differing compensating element 30
configurations. The compensated RFID tags of FIG. 17 are varying
combinations of "shorted" coils or loops of the antenna 24. That
is, different combinations of loops, or coils, in the antenna 24
were electrically connected, or shorted, to other loops of the
antenna 24. In this manner, the compensating elements 30 whose
response is shown in FIG. 17 are similar to those shown and
described above with respect to FIG. 3, in that they are
electrically connected to the RFID tag antenna, but they are not
part of the direct electrical path (circuit) traced from the inner
end of the RFID tag antenna to the outer end. In other words,
although the compensating elements 30 whose responses are shown in
FIG. 17 are electrically connected to the RFID tag antenna, they do
not form a part of the RFID tag antenna itself. The coils of the
antenna are numbered from 1 (the innermost loop or coil) to 9 (the
outermost loop or coil). The configurations of shorted coils and
the associated responses are indicated on FIG. 17 as follows:
TABLE-US-00001 Ref. numeral coils shorted 100 none 102 coil 1 to 2
104 coil 2 to 3 106 coil 4 to 5 108 coil 8 to 9 110 coils 1 and 2
to 3 112 coil 2 and 3 to 4 114 coil 3 to 4, coil 7 to 8 116 coil 7
and 8 to 9 118 coil 1 to 2, coil 3 to 4, coil 5 to 6, coil 7 to 8
120 coils 1, 2, 3 and 4 to 5
From FIG. 17 several generalizations can be made. For example,
shorting any one of the antenna loops to another antenna loop, from
the innermost to the outermost, produces a compensating element
effect. Also, shorting multiple coils, either to adjacent coils or
to non-adjacent coils, also produces a compensating element effect.
FIG. 17 indicates that many possible combinations of shorted coils
exist which can perform the compensation function. It shall be
understood, therefore, that these and other combinations of shorted
coils as compensating elements are within the scope of the present
invention.
Two different types of compensating elements 30 have thus been
described. One type is that described with respect to FIGS. 2-6, in
which the compensating element 30 is formed as physically separate
from the RFID antenna 24. This physically separate, compensating
element 30 can be electrically connected or electrically isolated
(i.e., not connected) to the RFID) antenna 24. Another type is like
that described above with respect to FIG. 17, in which physical
coils, or loops, of the antenna 24 itself are shorted to other
coils or loops of the antenna 24 to form a compensating element 30.
Each coil or loop fonning a continuous loop compensating element 30
is connected at a single point of electrical contact with a coil or
loop of the antenna 24. The selection of the type of compensating
element 30, namely, whether formed as a physically separate element
or formed as part of the RFID tag antenna itself, may depend upon
the specific application for which the RFID tags are to be used,
the desired resonant frequency of the compensated RFID tags, the
manufacturing techniques used to produce the RFID tags, and whether
the compensating element 30 is to be built into the RFID tag at
manufacture or added onto pre-existing, uncompensated RFID tags in
the manner discussed above with respect to FIG. 10.
FIG. 18 shows another application for compensated RFID tags 20.
FIG. 18 is a diagram showing a compensated RFID tag 20 on a
conductive substrate 160. The compensated RFID tag 20 comprises a
substrate 22, antenna 24, an optional RFID die (not shown), a
compensating element substrate 23, and compensating element 30. A
dielectric spacer 164 provides physical and electromagnetic
separation between the compensated RFID tag 20 and the conductive
surface 160 to be tagged. Again, for ease of illustration, the
compensating element 30 is shown located entirely within innermost
loop 24A of antenna 24. It shall be understood that the
compensating element 30 may take any one of several possible forms,
including any one of those shown and described above with respect
to FIGS. 2-12.
Compensated RFID tags 20 can be used for the tagging or labeling of
articles having metal and other conductive surfaces. Compensated
RFID tags 20 show improved performance compared to standard,
uncompensated RFID tags when the RFID tags are attached to metal or
other conductive surfaces and detected by a
magnetic-induction-coupled RFID system.
The read range, i.e., the distance at which an RFID reader can
detect and communicate with an RFID tag, can be used as a
quantifiable measure of RFID tag efficacy. In the presence of a
conductive surface 160, a compensated RFID tag 20 on a dielectric
spacer 164 shows greater read range than an equivalent
conventional, uncompensated RFID tag mounted on a similar
dielectric spacer on a similar conductive surface.
When labeling conductive surfaces, the effects of an electrical
"image current" distribution formed in the conductive plane in
response to the current distribution in an RFID tag should be taken
into account. When an RFID tag and the conductive plane are in
close proximity, the image current effectively negates the current
distribution in the RFID tag. The effect of the tag plus image
current can reduce the apparent tag response to the RFID reader,
which the reader may interpret as "no tag" present.
The dielectric spacer 164 separates the electrical currents in an
RFID tag from the induced image currents in the conductive surface.
The effective electrical thickness (the product of the physical
thickness, t, and the dielectric constant, .epsilon.) of the
dielectric spacer 164 may be increased either by increasing the
actual physical thickness, t, or by increasing the dielectric
constant .epsilon.. For applications where the RFID tag is to be
used as a label, a thick dielectric spacer may make the RFID tag
too thick to be a practical solution for marking articles. For the
compensated RFID tag on a conductive surface, a dielectric spacer
can be made from a dielectric material with a moderately low
.epsilon., such as .epsilon.<10 in one embodiment, or
.epsilon.<3 in another embodiment, for example. Examples of such
materials include foamed polymeric films, or hollow air-filled
glass or polymer bubbles in a low-.epsilon. matrix such as
polyethylene or polytetrafluoroethylene (PTFE), for example. The
thickness of the dielectric spacer 164 should be sufficient to
achieve the desired read range for the compensated RFID tag 20 on
the conductive surface 160. For example, a dialectric spacer having
a thickness t<10 mm, or having a thickness t<5 mm. Overall,
the compensated RFID tag 20 permits use of a thinner dielectric
spacer 164 so that the RFID label is less obtrusive.
The compensated RFID tags 20 result in improved efficacy of the
read function of the compensated RFID tag on conductive surfaces.
First, the compensated RFID tag provides longer read range in the
presence of a conductive surface for electrically thin dielectric
spacers. The compensated RFID tag on a conductive surface also
provides equivalent read range in a physically thinner
construction. In addition, the compensated RFID tag on a conductive
surface offers equivalent performance in a smaller tag, compared to
a conventional, uncompensated RFID tag.
Various embodiments of the invention have been described. These and
other embodiments are within the scope of the following claims.
* * * * *